OP,
Welcome to Flyertalk!
I've never had RNO not check my bag through on separate tickets, just took the agent a lot of time with the help desk (and once a big mess up with getting my passport into back into the system after my BP was issued and the gate scanner barfed (20 minute delay working with IT again) and almost didn't get on the plane even though my bags did). Most of my connections have been to OZ in LAX or BR in IAH or SFO and rare connections at ORD to OS usually leaving RNO in the afternoon. Obviously United is back to blowing off of their alliance obligations again or their IT staff can't bothered to deal with an issue that's been around for a very long time or non airport management doesn't want to actually make UA the Friendly Skies.
I'm always happy to work with the RNO crew especially when they know the issue is coming from higher up the food chain.